1. Field of the Invention
The invention relates to a fixing structure for a screwdriver's head, and more particularly, to a coupling tube that is durable and allows a more stable fixing of the screwdriver's head.
2. Description of the Prior Art
The screwing tools have been used for quite some time. Their versions and structures are different according to the manufacturing factories and application surroundings. A conventional coupling tube 11, as illustrated in FIG. 5, for receiving screwdriver's heads employs a sleeve 12 mounted on the coupling tube 11. The sleeve 12 is extendable and retractable by use of a spring 13. An engaging element 14 is disposed between the sleeve 12 and the coupling tube 11. Meanwhile, the engaging element 14 projects into an engaging seat 15 into which a screwdriver's head 16 is inserted. Accordingly, an engagement or a disengagement of the screwdriver's head 16 is achieved by the extending and retracting movement of the engaging element 14 when the sleeve 12 telescopically moves. However, the engaging element 14 of the prior art is constructed as round steel pearl, and this will cause much inconvenience in manufacturing and engaging the screwdriver's head 16. The drawbacks of the prior art are concluded as follows:    1. Bad engaging effect: Referring again to FIG. 5, the protrusion of the engaging element 14 is engaged into a V-shaped indentation of the screwdriver's head 16. As the engaging element 14 is a steel pearl, a full engagement can't be reached, thereby reducing the stability in screwing. In addition, the steel pearl and the V-shaped indentation will be damaged by the wearing action after a longer using period, thereby lowering its stability again.    2. Difficult manufacturing: In order to allow the extending and retracting movement of the engaging element 14 with the telescopic movement of the sleeve, the part of the coupling tube 11 for receiving the engaging element 14 has to be bored with a conic round hole that corresponds to the V-shaped indentation for engaging the steel pearl into the V-shaped indentation. Accordingly, the manufacturing requires a relatively high precision, thereby increasing the manufacturing difficulty and cost.